A new study reports that a vaccine-induced cellular immune response reduced simian immunodeficiency virus (SIV) levels in the semen of rhesus monkeys during the period of primary infection, a discovery that may ultimately aid in the fight against HIV-1 transmission in humans.
The researchers from Harvard Medical School, Boston, Massachussetts; Los Alamos National Laboratory, Los Alamos, New Mexico; and the National Institute of Allergy and Infectious Diseases, Bethesda, Maryland detail their findings in the October 2009 issue of the Journal of Virology.
HIV-1 replication peaks during the period of primary infection, therefore it is estimated that 50% of sexually contracted HIV-1 cases occur when the individual transmitting the virus is newly infected. No vaccines capable of inducing antibodies that completely neutralize a variety of HIV-1 isolates are currently available, however prior studies show that vaccine-elicited, virus-specific T-lymphocyte populations can limit viral replication during primary infection in nonhuman primates.
In the study two groups of rhesus monkeys were immunized with either vaccine vectors expressing SIV Gag/Pol or control vaccine constructs following which both were challenged intravenously with a highly pathogenic SIV quasispecies. Semen samples were collected from both groups of monkeys once a week for six weeks and then bi-weekly up to 16 weeks to evaluate both the acute and set point phase of SIV viremia. Analysis of the collected semen samples showed that SIV RNA was detected by day 7 in the control group and reached a peak at day 28 postinfection, while in the Gag/Pol group SIV RNA wasn't detected until day 14, reached a peak at day 21, declined to undetectable levels by day 42 postinfection and remained that way for the duration of the study. Significantly, the peak virus RNA level in the semen of the Gag/Pol group was found to be lower than the control group and it remained substantially lower from days 28 to 80 postinfection.
"The demonstration in the present study of a direct association between vaccination and diminished seminal plasma SIV RNA levels suggests that vaccination may be an effective measure for reducing HIV-1 transmission," say the researchers.
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